The magnitude of currents distributed in electrical power systems are often in the order of hundreds or thousands of amperes. It is not feasible to connect current measuring monitoring or protection devices directly to such high currents. Therefore, current transformers are used to transform the currents to magnitudes applicable to said monitoring or protection devices
These transformers are not ideal, however. Above certain current levels they exhibit saturation phenomena which distort the transformation.
Such saturation will in most cases occur when a current being monitored in the system does not behave as anticipated. Such deviating behaviour may also occur because a fault occurs in the system. It is furthermore often in this type of situation that a correctly detected property of the current is most important.
Most current measuring, monitoring and protection devices have to deal with this imperfection in one way or the other. Some devices that may suffer a lot because of this are current measuring protective devices. They are assigned to take action based on the current during the period of time when the saturation phenomenon may be in its worst state.
It is common in protective devices to have a current signal filtered by a pair of filters that reveal the phasor representation of the signal; that is, a complex, magnitude/phase, representation of the signal. The phasor is typically the operative quantity for various functions within the protective device. The current transformer saturation may cause significant errors in the phasor estimation, unless sufficient measures are taken.
The nature of the current transformer saturation phenomenon is such that, during the intermittent periods when it occurs, the transformed (secondary) current waveform deviates significantly from the non-transformed (primary) current waveform, which it is expected to reproduce.
The normal way to handle this is through trying to fully restore the deviating portions of the secondary current waveform to correctly replicate the primary current waveform, thereby providing a secondary current signal that is apparently not affected by the saturation. Conventional filtering of this restored signal, in order to obtain a phasor representation, will therefore not be associated with any problem related to the current transformer saturation.
One technique with the aim of fully reconstructing the secondary current waveform is described in WO93/13581, where the current waveform is partly restored through modelling the current transformer behaviour.
Other prior art documents describing this and similar approaches are U.S. Pat. No. 6,072,310, US2005/0140352 and U.S. Pat. No. 6,040,689.
An alternative technique is based on Artificial Neural Networks. The technique includes “training” of the neural network. This is for instance described in EP 0 980 129. Training of neural networks may however be impractical in a commercial application.
The perhaps most viable of complete signal reconstruction methods is based on a signal model alone, such as described by Kang et al. in “A compensation Algorithm for the Distorted Secondary Current of a Current Transformer”, Eighth IEEE International Conference on Development in Power System Protection, 2004, page 140-143.
Typically signal reconstruction may be based on autoregression. One document describing such a technique is “Autoregressive Model-based Compensation Method for the Saturated Secondary Current of a Current Transformer”, D-G Lee at al., Proceeding (521) European Power and Energy Systems, 2006, page 287-291.
DE 19928192 describes a method for reconstructing a whole signal waveform using unsaturated current samples, apparently by using detected extreme points from the unsaturated part of the signal.
All these techniques, aiming at fully reconstructing the secondary waveform, are more or less demanding with respect to digital processing resources. The processing power is in many situations limited and it may be desirable to use this limited processing capability to other more urgent uses such as detecting a fault, determining type of fault, determining distance to fault as well as determining various corrective actions to a fault.
Hence, there is a need for techniques that can lessen the negative impact of current transformer saturation on phasor estimation while at the same time limiting the required processing resources.